1. Field of the Invention
The present invention relates to an electrically erasable programmable read-only memory (EEPROM) and, more specifically, to an EEPROM for storing multi-value information of more than one bit in a single memory cell.
2. Description of the Related Art
A memory cell having a MOSFET structure wherein a charge storage portion and a control gate are formed one on another on a semiconductor substrate, is known as one memory cell of an EEPROM. The memory cell usually stores data "0" or "1" in accordance with the number of charges stored in the charge storage portion, that is, it stores data of one bit in one cell. A multi-value data storing system for storing data of two bits or more in a single cell, is known to achieve a higher packed density type EEPROM. For example, in a 4-value data storing system, four different numbers of charges are stored in a charge storage portion in order to store data "0", "1", "2" and "3" in a single cell.
An example of the states of data stored by the 4-value data storing system will now be described. In this example, the following states are defined. The state in which the number of charges stored in a charge storage portion is zero is a neutral one, while the state in which a positive charge is stored in the neutral state is an erase one. The erase state corresponds to data "0". For example, an erase operation is performed when a high voltage (up to 20 V) is applied to a substrate and a control gate is set at 0 V. The states in which negative charges are stored in the neutral state are a data "1" state, a data "2" state and a data "3" state. The number of negative charges in the data "2" state is larger than that in the data "1" state and smaller than that in the data "3" state. The negative charges are stored in the charge storage portion by setting the substrate, source and drain to 0 V and applying a high voltage (up to 20 V) to the control gate.
Data stored in an EEPROM is generally assured for ten years; however, the charges stored in the charge storage portion leak very gradually. This leak makes data "0", "1", "2" and "3" to be confused with one another, so that the number of charges corresponding to each data is usually set discretely. The difference in the number of charges is called a charge number margin.
If the charge number margin is too small, data cannot be held for ten years. On the other hand, if the margin increases, the number of charges corresponding to, e.g., data "3" has to increase. For this reason, write time is lengthened and write voltage is heightened as compared with a normal memory cell for storing one-bit data. The larger the number of stored charges, the larger the number of leaking charges. Thus, the data hold period is not so lengthened that the charge number margin is increased.
An EEPROM to which data of plural bytes are written at once is known. This memory includes a data circuit for temporarily storing data of plural bytes in order to shorten the write time. To store multi-value data in the EEPROM, the data circuit has to be designed so as to store the multi-value data, with the result that the data circuit is increased in area.
As described above, the multi-value data storing system is effective means for achieving a high degree of integration, but has the following problems:
(1) The reliability of data assurance is lowered; and
(2) The control circuit such as a data circuit excluding a memory cell is increased in area.